In the so-called developing nations, wind turbines are commonly used to drive irrigation pumps and the like. Recently, there has been a resurgence of interest in the industrial nations as well with respect to these machines, due to the rise in cost of fuel and electricity.
Applicant has been involved for a number of years in the design and development of wind turbines. In this connection, there has always been an impetus to achieve efficiency but with simple structures which can be manufactured or repaired at the village level.
In earlier work, applicant produced a blade having a delta-wing and flap extension integral therewith. The delta-wing and flap extension are angularly arranged relative to each other at an angle of about 25.degree.. This cupped blade is attached to a rigid spoke which extends radially from the turbine rotor.
This blade was found to be particularly efficient and simple to manufacture.
Applicant has disclosed the blade in a paper entitled "An axial-flow wind-turbine with delta-wing blades", published in "Alternative Energy Source", Volume 4, Hemisphere Publishing Corporation (1977).
As development work on the blade proceeded, applicant sought to utilize a single skin construction, but was confronted with the need to reinforce it.
Tests were performed to establish whether the reinforcing ribs, which were arranged in parallel arrays for structural convenience, should be secured to the suction or rear surface of the blade and protrude perpendicularly therefrom or, alternatively, the opposite (pressure) surface.
It was found, from wind-tunnel testing, that the attachment of the protruding ribs to the suction surface disrupted the double helical vortex flow pattern on said suction surface and interferred with the rotor performance. It was discovered that far less flow disruption occurred when the reinforcing ribs were attached to the pressure surface. The disturbance was further found to be minimal when the rib arrays were substantially aligned with the local flow direction.